1. Introduction
The field of nanotechnology has witnessed considerable progress in recent years, both in design and the scope of applications. As an interdisciplinary field covering diverse research areas, nanotechnology has undoubtedly led to industrial production of novel nanomaterials with designer properties such as being more robust, lighter, longer-lasting, anti-reflective, antimicrobial, electrically conductive, or becoming more luminous. In particular, nanoparticle (NP)-based medicine has gained traction, promising to revolutionize medical treatment with innovative therapeutics that are more potent and less toxic1.
Nanotechnology is revolutionary, and its hype is justified, especially for improving the quality of human life with novel consumer products through various materials and manufacturing methods. However, there are concerns on nanotechnology potentially creating delayed impacts on the environment and human health, especially where detrimental consequences are only noticed after commercialization has long begun. Thus, technologies being developed require cautionary measures to be upheld to avoid future predicaments for the environment and humanity, in tracking towards a sustainable future. This is to prevent history from repeating itself, such as petrol with lead, electronics with polychlorinated biphenyls, chlorofluorocarbons reducing upper atmosphere ozone dramatically, and the construction material asbestos, all of which led to environmental disasters.
Despite substantial research in the field and considerable progress, strategies for manufacturing nanoscale materials, through both top-down and bottom-up production processes, still face challenges. Top-down approaches to reduce more extended material to nanoscale dimensions often use a number of materials leading inevitably to waste generation, and are inappropriate for several materials2. Traditional assembly lines create products by building them up from the molecular level, the bottom-up strategy, which combines chemical synthesis and self-assembly. For current practical abilities, the main challenges are that the bottom-up strategy can be time-consuming, requiring extensive expertise and skill to control the size, morphology, and properties of the nanoscale products. Of paramount importance is the choice of synthetic method to finely control these features while circumventing uptake of impurities2. As such, the fundamental problem regarding the bottom-up strategy is developing the capability to exquisitely control the synthesis of the NPs while appropriately controlling size, morphology, and properties at nanoscale dimensions.
From a technological viewpoint, traditional methods abound in developing processes to control the growth and properties of materials. Such bottom up material processing at the nanoscale dimension has been developed using channel-based microfluidic devices, albeit with some limitations. The main drawback is insufficient mixing resulting from laminar flows, often requiring sample dilution or reagent homogenization. The mixing process is usually restricted to diffusion control processing under such flows, denying the possibility of harnessing the advantages of turbulent mixing available in macro-scale systems3. While mixing enhancements can be achieved by incorporating multiple system parameters, including energy input, the velocity of flow, and the geometry of the mixers, these methods are time-consuming, leading to cost inefficiencies. In addition, channel-based microfluidic devices can suffer from clogging, specifically in the processing of macromolecules or at high reactant concentrations. Furthermore, incorporating external fields, such as electrical, magnetic, and laser fields, to control the processing is inherently complex for channel-based microfluidic platforms4. Although other mechanical energy forms, including sonication and grinding or milling, are effective in materials processing, they suffer from indiscriminate events in time and place, such as in cavitation and uneven energy transfer, resulting in non-uniform products. This can generate waste that, coupled with high energy usage, limits the sustainability metrics of such processing. A paradigm shift in microfluidics design is required to overcome such limits.
Thin film processing is an emerging technology where the liquid is subjected to centrifugal forces/shear stress or mechanical energy within dynamic thin films on a surface. These forces are useful in a range of thin film vortexing technologies, including for chemical synthesis and separations5, material synthesis6, material processing, lab-on-a-disc microfluidics7 and enabling chiral selection8. Thin film processing offers several advantages, including accelerated reaction kinetics and improved control over chemical reactivity. The application of shear stress presents opportunities for enabling new types of chemical reactions and generating materials with new shapes, morphologies and sizes.9 Thin film processing holds potential in situations where traditional batch processing is impractical or when conventional methods fail to provide access to unique forms of materials.10
Rotary devices that utilize centrifugal forces, pushing away from the rotation axis, are prime examples of how these forces can shape interfaces and effectively control material synthesis and chemical reactivity9. A diverse range of rotary devices have been reviewed and shown to achieve such performances, including lab-on-a-disc system11, spiral seperators12, spinning disc reactors13, and vortex fluidic devices14. This review introduces the vortex fluidic device (VFD) as a paradigm shift in flow processing, with scalability factored in under the continuous-flow mode of operation of the device, along with its utility for tuning the size, morphology, and properties of materials at the nanoscale dimension. The VFD delivers high shear as a constant form of mechanical energy, with tunable control over the processing. Processing in the device is not limited by diffusion control, providing a route to kinetically trapped novel forms of nanomaterials, and processing can also be facilitated by applied external fields for which the microfluidic platform is suited with the thin film of liquid approaching uniformity of treatment as such. Continuous-flow processing of the VFD is directly scalable, unlike batch processing which requires precise process engineering for upscaling, in overcoming uneven mixing and heat transfer. Whereas previous reviews of VFDs have focused on chemical transformations15 and comparisons with other microfluidic devices16, this review aims to deliver additional information about the significance of utilizing the VFD to transform material structure-property relationships at the nanoscale with emphasis on its high green chemistry metrics.